The mechanisms(s) by which the mammalian kidney maintains (autoregulates) renal blood flow (RBF) and glomerular filtration rate (GFR) within such narrow limits as blood pressure (BP) is acutely raised toward 180 mm Hg or lowered toward 60 mm Hg is not well understood. Single nephron glomerular filtration rates (SNGFR) and proximal tubule (PT) pressures will be measured in the same kidney at high BP and again at 2 lower BP within the range of BP where autoregulation occurs. Samples of tubular fluid will be examined by electron probe analysis and helium glow photometry in order to determine if compositional changes in tubular fluid ions are responsible for the adjustment in afferent arteriolar tone which accompanies changes in BP that allow RBF and GFR to remain constant. In addition, tubular perfusion of the nephron will be employed to change the ions delivered to the distal nephron over a much wider range. Again SNGFR and PT pressures will be measured.